The present invention relates to a calibration dummy or reference standard for simulating animal or human cell tissue, for purposes of calibrating, adjusting or, more generally, testing ultrasonic equipment.
It is known to employ ultrasonics for purposes of medical diagnostics. Particularly ultrasonic pulses are transmitted to the body and tissue boundaries produce reflection of these pulses. The transit time of such pulses permits the determination of the depth of such a boundary.
Generally speaking, the acoustic impedance of a material is the product of its density .rho. and of the speed c of acoustic waves therein. Whenever a beam of acoustic waves such as ultrasonic waves passes through a boundary defined by a steep change in the acoustic impedance, a portion of the vibration energy is reflected, while the remainder passes through the boundary. The reflected signal portion or echo is detected and the transit time, i.e. the period between launching of the transmitted signal and the receiving of the echo can be used to generate a so called A-image (see Bergmann, L., "Der Ultraschall", S. Hirzel-Verlag, Stuttgart 1954; Matauschek, J., "Einfuhrung in die Ultraschalltechnik", VEB-Verlag Technik, Berlin 1961, and Krautkramer, J. and Krautkramer, H., "Werkstoffprufung mit Ultraschall", Springer-Verlag 1975.) This transit time is also directly proportional to the thickness of the tissue which can be calculated directly therefrom, if, in fact, the speed of acoustic waves in the tissue is known.
Aside from the echos being produced on boundaries or impedance changes, the tissue itself absorbs part of the ultrasonic energy, whereby, however, the absorption depends on the frequency of the sonic or ultrasonic signal. Nevertheless, similarly structured boundaries and layers but being located deeper in the tissue will produce only weak echos on account of the aborption of the incident as well as of the reflected signal. In order to compensate for this reduction in amplitude, one has introduced a technique which can be described as transit time dependent amplification or depth compensation. Since a long transit time goes hand in hand with a weak echo signal, one will increase the amplifier gain the longer the delay of the expected or actually appearing echo, metered from the launching time of the test pulse.
This depth compensation has been provided for empirically. One simply selected the proper amplification on the basis of empirical tests and trial and error methods, i.e. on a highly subjective basis. As far as we know, no tissue-dummy or reference standard has yet been suggested by means of which one can objectively calibrate and adjust ultrasonic testing equipment in order to compensate the declining amplitude of echos being returned from deeper boundaries of and in the cell tissue.